A term female newborn has ambiguous genitalia with clitoromegaly and fused labioscrotal folds. On day 7 she develops vomiting, poor feeding, hyponatremia of 128 mmol/L, and hyperkalemia of 6.4 mmol/L. Which enzymatic block explains both findings?

• A 11-beta hydroxylase deficiency
• B 21-hydroxylase deficiency ✓
• C 17-alpha hydroxylase deficiency
• D 5-alpha reductase deficiency

Explanation

21-hydroxylase deficiency is the commonest form of congenital adrenal hyperplasia. The block diverts steroid precursors into the androgen pathway, virilizing a genetic female, while simultaneously preventing cortisol and aldosterone synthesis. Aldosterone loss causes the salt-wasting crisis with hyponatremia and hyperkalemia seen here. 11-beta hydroxylase deficiency causes virilization too, but its retained mineralocorticoid precursors produce hypertension, which contradicts this child's salt wasting. 17-alpha hydroxylase deficiency causes undervirilization and hypertension.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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